Get a mirror if you don't already have one so you can see what's coming up from behind. When I see someone about to pass me and not giving me my 3' I extend my arm and wiggle my fingers it works for me most of the time.
I also went to some training class the local Baton Rouge Bike Club offered and went for some club rides, really helped my confidence.
That said today in New Orleans was the closest I have come to being hit I was in the bike lane and a parked car decided to ignore me and enter the lane a little hard breaking and a loud hey watch out did the trick. Also my 1200 lumin blinking light also seemed to grab there attention.
That said it really was not scary, the car was not going to hit me I was aware and had room to maneuver.